This invention relates to a process for crystallizing, by distillation, a basic zinc carbonate (2ZnCO.sub.3.3Zn(OH).sub.2 or ZnCO.sub.3.3Zn(OH).sub.2.H.sub.2 O) from a solution of a basic zinc ammonium carbonate (Zn(NH.sub.3).sub.4 CO.sub.3).
Waste zinc is discharged from a Zn plating plant such as in a steel mill. However, zinc has various uses and is used, for example, as a pigment or a vulcanization accelerator, in the form of ZnO. Therefore, if the waste zinc is treated to remove impurities such as heavy metals introduced during a process where the waste is produced and the zinc recovered, it would be very advantageous.
By this reason, the inventors of the present invention have previously proposed, in Japanese Patent Laid-Open (Kokai) No. 59-88319, a process for recovering zinc cabonate from zinc-containing substances.
To recover zinc carbonate from the zinc-containing substances, it is preferable to dissolve the zinc-containing substances in a solution which contains at least NH.sub.4 OH and (NH.sub.4).sub.2 CO.sub.3, and then crystallize zinc carbonate from the solution. As the waste zinc materials contain heavy metals as described above, it is preferable to remove the heavy metals for recovering zinc compounds of high quality. In this case, the uses of the recovered zinc will be increased.
The inventors found, after an intensive and extensive study, that it is optimum to use a procedure comprising dissolution.fwdarw.ion exchange.fwdarw.crystallization to remove the impurities as much as possible. This process includes the following reactions:
Dissolution Step EQU Zn+(NH.sub.4).sub.2 CO.sub.3 +2NH.sub.4 OH.fwdarw.Zn(NH.sub.3).sub.4 CO.sub.3 +H.sub.2 .uparw.+2H.sub.2 O (1)
Ion Exchange Step EQU Me.sup.2+ +Zn.fwdarw.Me.dwnarw.+Zn.sup.2+ ( 2)
Crystallization Step EQU 5Zn(NH.sub.3).sub.4 CO.sub.3 +3H.sub.2 O.fwdarw.2ZnCO.sub.3.3Zn(OH).sub.2 .dwnarw.+3CO.sub.2 .uparw.+2ONH.sub.3 ( 3A) EQU 4Zn(NH.sub.3).sub.4 CO.sub.3 +4H.sub.2 O.fwdarw.ZnCO.sub.3.3Zn(OH).sub.2. H.sub.2 O.dwnarw.+3CO.sub.2 .uparw.+16NH.sub.3 .uparw. (3B)
In the step of crystallization, 80 to 90% of the crystallization is effected by reaction 3B. Little crystallization occurs in reaction 3A.
To carry out the crystallization step, it is common in the field to employ one or more crystallizers.
When the crystallization step is carried out by the crystallizers according to the conventional technique, it is preferred to operate the crystallizers like a multiple effect evaporator. For example, steam is fed into a first effect and the resultant vapor is sequentially introduced into the succeeding effects to evaporate and distill ammonium. In general, a considerable number of crystallizers and a considerable amount of steam are needed to lower the ammonium content to, for example, 1000 ppm or less. Besides, the operation is of a simple distillation and batchwise. Thus, continuous operation can not be carried out.
Whereas, the inventors have found that the crystallization step, as specified, above can be carried out by using a distillation column. This is not in accordance with the conventional knowledge of the field.
In this connection, it is to be noted that when the crystallization is carried out bY using a distillation column, if a material solution to be subjected to distillation-crystallization has a Zn concentration of 10 wt % (hereinafter, the percentage represents a percent by weight) or more, the solid concentration in the product slurry becomes 20% or more. Since the volatility of ammonia is high and the evaporation amount of ammonia is large at upper plates of the distillation column, due to the evaporation characteristics of ammonia, zinc carbonate is violently crystallized at that portion. As a result, there occurs heavy sticking of the crystals to the walls or plates inside the distillation column, and bubble-cap portions or the like become clogged, which necessitates stopping of the operation and washing thereof, for example, only after an operation of one day or so.
For example, sticking of crystals X occurs as shown in FIG. 10 in the case of a sieve-tray column, and as shown in FIG. 11 in the case of a bubble-cap tower.
The inventors have attempted to prevent sticking of the crystals by changing the materials or configuration of the distillation column, but they have found that these measures will not be an essential solution for prevention of the crystal sticking. The inventors have further found, in their further study of prevention of the crystal sticking, that the sticking of crystals occurs, in crystallization by the distillation column, when the concentration of the starting solution is high. Based upon this finding, it was concluded that it is effective to prevent the sticking of crystals to provide a plurality of crystallizers before the distillation column so as to partiallY effect the crystallization operation by these crystallizers for lowering the concentration of the feed solution.
However, the general distillation operation in the distillation column is such that distillation is effected at each plate or tray where the material solution comming down therethrough is brought into contact with vapor. In such an operation, the amount of the solution retained by each of the plates or trays is rather small, and if crystallization occurs in such an amount of solution, sticking of the crystals to the plates or trays can not be avoided.
On the other hand, an indirect-type heating means using steam as a heating source is generally employed for carrying out a distillation operation. In this case, however, the heating means becomes inoperative, for example, only after an one-hour operation, because of the sticking of the resultant crystals to the heating faces. As a countermeasure against this, two sets of heating means may be provided so that washing liquid is introduced into one of the heating means for removing the sticking crystals, while the other is in operation. This measure, however, requires too much trouble to continuously operate the distillation system.
In the case where the crystallization is carried out only by the crystallizers, the obtained crystals are of a poor quality having a low bulk specific gravity and a tree-like crystal structure with whiskers (observed by electron microscope). Further, the resultant crystals have a high Cl ion content which is not desirable for some uses. In addition, the resultant crystals have a large surface area and contain a large amount of surface water due to the crystal structure as described above, so that the efficiency of filtration and washing of the succeeding process is not good.